This invention relates generally to friction drives of the reduction type, and more particularly to an improved friction drive of that type employing a compliant rotary member as either a drive or a driven member thereof.
In conventional friction drives of the reduction type generally noncompliant members are employed as both the drive and the driven members. Such drives typically require more parts, more precision of concentricity and diameter, and may have more backlash than desirable for many applications.
Accordingly, it is the principal object of this invention to provide an improved friction drive of the reduction type having fewer parts and no backlash.
Another object of this invention is to provide a friction drive of the reduction type, as in the last object, that may be employed for driving a stage of the precision step-and-repeat alignment and exposure system.
Another object of this invention is to provide a compliant rotary member that may be employed, for example, as either a drive member or a driven member in a friction drive of the reduction type.
Still another object of this invention is to provide a compliant rotary member that, when employed as in the last object, makes the precision of concentricity and diameter of both the drive and the driven members less critical than would be the case if a noncompliant rotary member were employed.
These objects are accomplished according to the illustrated preferred embodiment of this invention by mounting a motor-tachometer on a first support member, by providing the motor-tachometer with a hardened rotary drive shaft, by mounting a rotary driven shaft on a second support member, and by mounting a compliant rotary member of greater diameter than the hardened drive shaft on the driven shaft in abutment with the drive shaft. The compliant rotary member comprises a central portion coupled to an outer portion by three springs symmetrically arranged around the periphery of the central portion and each rigidly attached at one end to the inner portion and at the other end to the outer portion. In assembling the friction drive, the first and second support members are secured together so that the rotary compliant member is forced against the hardened drive shaft and so that the outer portion of the rotary compliant member is eccentrically offset with respect to the central portion.